Extremely thin articles of glass, such as those made from fused silica are used in a number of industries. For example, manufacture of chips used for analyzing DNA material requires fused silica plates thinner than about 0.70 mm. Fused silica plates having thickness between 0.30 to 0.80 mm are particularly suitable for use as pellicles to protect photomasks, which are used in the lithography process for manufacture of semiconductors, from air-borne contaminants. Besides their thinness, these photomask pellicles are large area substrates, typically between 15 cm×15 cm and 25 cm×25 cm in size. As specified by the semiconductor industry, these pellicles also should be made of high purity silica, so that they exhibit high optical transmittance at the ultraviolet wavelengths of interest to the industry, and also so that their transmittance levels do not quickly decrease under prolonged exposure to ultraviolet irradiation.
Published U.S. Patent Application No. 2002/0098420 A1 to Eynon describes a photomask assembly requiring use of a fused silica pellicle. However, the application does not disclose a method of manufacture for such a pellicle. Currently, such thin fused silica plates are made by cutting thick fused silica blanks into thin plates, and then mechanically grinding and polishing these plates to the desired thickness and surface finish. However, the cutting and grinding of silica blanks to produce thin fused silica pieces are difficult processes, due to the fragility and hardness of the silica glass. As a result, this method of manufacture is expensive and time-consuming.
Published Japanese Patent Application No. 2001-154340 to Shibuya et al. discloses an alternative process involving hardening of a thin polysilazane-based polymer on a PET film. The steaming and calcination of this film yield a silica pellicle having a thickness ranging between 0.01 μm and 50 μm.
These thin silica articles also can be fabricated using a sol-gel process, by casting a sol in a mold having a predetermined thickness, gelling this sol, and drying and densifying the gel to obtain a thin silica glass article. Published Japanese Patent Application No. 63-129028 to Yajima et al. describes a sol-gel process in which a silica sol is cast between two parallel plates, gelled, dried, and densified at 1,500° C. to produce glass plates having thickness of 4.0±0.01 mm. Published Japanese Patent Application No. 61-236620 to Mizuno et al. discloses casting, gelation, and drying of silica-based sols on an immiscible liquid, followed by densification of the dried gels at 1,500° C., to obtain glass plates having thickness of 0.95 mm. Published Japanese Patent Application No. 04-154638 to Ando et al. discloses casting a silica-based sol on a flat silicon sheet, followed by gelation and drying of these sols. Densification of these dry gels yields thin-film silica glasses. Published Japanese Patent Application No. 01-096024 to Nakajima describes gelation of a silica-based sol under centrifugal force, followed by drying and densification of the sols to yield thick-walled flat glass.
Thin silica plates having smooth surfaces can be obtained by applying the method disclosed in the Shibuya et al. application or any one of the above-described sol-gel processes. However, such thin articles of silica have been found to typically exhibit substantial warpage during their preparation, particularly during the step of densification (i.e., sintering) to form the glass. This densification generally takes place at temperatures between 1,000° C. and 1,500° C. Particularly for use in large area pellicles, this warpage is considerably beyond the specified limits of the photomask industry. The above-described publications do not disclose a method to prevent such warpage.
It should be appreciated from the foregoing description that there remains a need for a cost-efficient method for fabricating high-quality thin glass articles free of substantial warpage. The present invention fulfills this need and provides further advantages.